U.S. Pat. Nos. 4,640,352 and 4,886,118 disclose conductive heating of subterranean formations of low permeability that contain oil to recover oil therefrom. Such low permeability formations include oil-bearing diatomite formations. Diatomite is a soft rock that has very high porosity but low permeability. Conductive heating methods to recover oil are particularly applicable to diatomite formations because these formations are not amenable to secondary oil recovery methods such as water, steam, or carbon dioxide flooding. Flooding fluids tend to penetrate formations that have low permeabilities preferentially through fractures. The injected fluids therefore bypass a large amount of the hydrocarbons in the diatomite formations. In contrast, conductive heating does not require fluid transport into the formation. Oil within the formation is therefore not bypassed as in a flooding process.
Vertical temperature profiles will tend to be relatively uniform when the temperature of a formation is increased by conductive heating. This is because formations generally have relatively uniform thermal conductivities and specific heats. Transportation of hydrocarbons in a thermal conduction process is by pressure drive, vaporization, and thermal expansion of oil and water trapped within the pores of the formation rock. Hydrocarbons migrate through small fractures created by the expansion and vaporization of the oil and water.
Considerable effort has been expended to develop electrical resistance heaters suitable for injecting heat into formations having low permeability for thermal conductive heating of such formations. U.S. Pat. Nos. 5,065,818 and 5,060,287 are exemplary of such effort. U.S. Pat. No. 5,065,818 discloses a heater design that is cemented directly into a formation to be heated, eliminating the cost of a casing in the formation. However, a relatively expensive cement such as a high-alumina refractory cement is needed.
Gas-fueled well heaters which are intended to be useful for injection of heat into subterranean formations are disclosed in, for example, U.S. Pat. Nos. 2,902,270, and 3,181,613 and Swedish Patent No. 123,137. The heaters of these patents require conventional placement of casings in the formations to house the heaters. Because the casings and cements required to withstand elevated temperatures are expensive, the initial cost of such heaters is high.
U.S. Pat. No. 5,255,742 (application Ser. No. 896,861 filed Jun. 12, 1992) and application Ser. No. 896,864 filed Jun. 12, 1992, now U.S. Pat. No. 5,297,626, respectively, disclose fuel gas-fired subterranean heaters. The heaters of this patent and patent application utilize flameless combustion to eliminate hot spots and reduce the cost of the heater, but still use high alumina refractory cements to set the burner within the formation.
It is therefore an object of the present invention to provide a method to inject heat into a subterranean diatomite formation utilizing a heater within a wellbore wherein the thermal conductivity of the formation in the vicinity of the wellbore is enhanced over the thermal conductivity that could be obtained by sintering the formation only at the long-term heater operating temperatures.